Technical Field
The present invention generally relates to the formation of solder balls on a wafer and, more particularly, to the formation of such solder balls using injection molding.
Description of the Related Art
As fabrication processes reach density limits in the fabrication of two-dimensional (2D) integrated circuits, three dimensional (3D) and two-and-a-half dimensional (2.5D) packaging processes are gaining traction due to their ability to provide high bandwidth and short transmission lengths between devices. 2.5D packaging processes stack multiple layers of 2D circuits, each having their components arrayed in a single horizontal plane. An intermediary layer provides vertical interconnects between the vertically stacked device layers. True 3D packages, meanwhile, have one die stacked directly on top of another die, with through-silicon vias providing communications between components at different vertical levels.
In each case, dies are mounted to one another with electrical interconnections. This may be accomplished by forming solder bumps on the surface of one or more of the dies to be bonded prior to the bonding. One die is then positioned over the other (e.g., in a “flip-chip” process) and the solder is reflowed, creating the appropriate electrical connections.
However, the joining pitch and bump size need to be very fine for 3D and 2.5D packages compared to conventional flip-chip bonding. This results in a challenging fabrication process that sometimes results in join failures due to stress concentration at the joining area and at the joining interface, as well as electromigration failures due to high current densities.
One form of packaging is wafer-level packaging, which packages an integrated circuit while the chip is still part of the wafer. After the packages have been formed, they may be separated from one another by cutting the wafer. In such a process, the interconnections and solder bumps are formed for many chips all at once.
Existing processes for forming solder balls include screen printing, where a reusable metal mask is placed on a die before solder is printed on, and solder ball dropping, where pre-formed solder balls are physically positioned in the reusable mask. Another approach uses electroplating or electroless plating to form solder on an exposed seed metal over contact pads for the die. In the case of screen printing and solder ball drop processes, there is a risk of void creation as flux material becomes a gas during solder reflow. Pre-forming solder balls also adds significant additional expense and, furthermore, the solder balls are often incompletely distributed. Solder plating, meanwhile, can be too time consuming to be practical for large-scale fabrication.